Sync Gateway compatibility matrix is confusing

Reading the Couchbase Server compatibility section of the docs begins to suggest that various versions of Sync Gateway are compatible with versions of Couchbase Server they were likely never tested with.

This table suggests that 1.4 of SG works with Couchbase Server 7.1 when I do not believe these versions are ever tested against one another with 1.4 going EOL in 2019. But if this is the case that it's meant to represent the feature introduced in 1.4 is available in these server versions, then we should instead introduce an actual compatibility matrix similar to what SDKs have to show which versions we test again.